Permanent magnets based on compositions containing iron, neodymium and/or praseodymium, and boron are now in commercial usage. These magnets contain grains of tetragonal crystals in which the proportions of transition metal (TM), rare earth (RE), and boron are exemplified by the empirical formula RE.sub.2 TM.sub.14 B.sub.1 and where at least part of the transition metal is iron. These magnet compositions and methods of making them are described in U.S. Ser. No. 414,936, filed September 3, 1982, and U.S. Ser. No. 544,728, filed October 26, 1983, both assigned to the assignee of this application and incorporated herein by reference. The grains of the tetragonal crystal phase are surrounded by a small amount of a second phase that is typically rare earth rich and lower melting compared to the principal phase.
A preferred method of making magnets based on these compositions is the rapid solidification of an alloy from a melt to produce very fine grained, magnetically isotropic particles. Melt spinning or jet casting is an efficient method of producing rapidly solidified ribbon flakes which may be directly quenched to near optimum single magnetic domain size or overquenched and heated to promote suitable grain growth. The flakes can be ground to a convenient size for further processing.
It is also known that fine grained RE-TM-B particles can be hot pressed and/or hot worked and plastically deformed to form isotropic and anisotropic permanent magnets with exceptionally high energy products. This practice is described in U.S. Ser. No. 520,170, filed August 4, 1983, assigned to the assignee of this application and is incorporated herein by reference.
A typical hot processing practice entails overquenching an alloy of a preferred RE-TM-B composition such as Nd.sub.0.13 (Fe.sub.0.95 B.sub.0.05).sub.0.87. The thin, friable ribbon is then crushed or ground into particles of a convenient size for an intended hot pressing operation (50 -325 mesh, e.g.). Rapidly solidified ribbon particles are stable in air at room temperature. The particles are heated in a nonoxidizing atmosphere to a suitable elevated temperature, preferably about 650.degree. C. or higher, and subjected to pressures high enough to achieve a magnetically isotropic, nearly full density compact or a magnetically anisotropic plastically deformed compact. U.S. Ser. No. 520,170teaches that processing may be accomplished by hot pressing in a die, extrusion, rolling, die upsetting, hammering or forging, for example. Hot isostatic pressing is useful to make fully dense isotropic magnets, but has a slow cycle time.
These processes are all useful to form moderately sized magnets into simple shapes. This application relates particularly to a novel method of hot forming and/or hot working rare earth-transition metal powders or compacts to make relatively large permanent magnets with consistent densities and magnetic properties. Such large magnets could be economically cut into smaller shapes or used for applications where several magnets must otherwise be pieced together with some sacrifice of magnetic properties.
As used herein, the term "working" shall mean the application of heat and pressure to a workpiece to cause material flow therein which induces magnetic anisotropy in substantially amorphous to very finely crystalline RE-TM-B alloys. The term "forming" shall mean the application of heat and pressure to a workpiece to cause consolidation thereof and may or may not include working.